The present invention concerns an ultrasonic oscillating unit comprising a converter and a sonotrode optionally connected to the converter by way of an amplitude transformer, wherein the sonotrode has a sealing surface which is substantially in the form of a cylinder surface and the ultrasonic oscillating unit has a holder for fastening the ultrasonic oscillating unit to a machine stand of an ultrasonic welding apparatus.
Ultrasonic welding is a method of joining plastic materials. Ultrasound is a mechanical oscillation above the audible limit. The frequency range begins at about 20 kHz and extends to frequencies of 1 GHz. Such ultrasonic frequencies are frequently generated by means of piezoelectric sound transducers (converters) from electrical energy. That mechanical oscillation energy is applied to the workpiece or the material to be processed by way of the sonotrode which is connected to the converter, possibly by way of an amplitude transformation member (booster). The surface of the sonotrode which is intended to come into contact with the material to be processed is also referred to as the sealing surface.
The ultrasonic oscillating unit thus represents a structure which oscillates in operation and consists of the converter, optionally the amplitude transformation member and the sonotrode.
To effectively transmit the ultrasonic oscillation by means of the ultrasonic oscillating unit it is necessary to cause the ultrasonic oscillating unit to assume a resonance condition. In dependence on the structure of the ultrasonic oscillating unit it has a multiplicity of natural frequencies. It is only when the converter generates a natural frequency of the ultrasonic oscillating unit that resonant oscillation of the oscillating unit occurs. Therefore the converter and the ultrasonic oscillating unit must be tuned to each other.
Strictly speaking the resonance frequency differs somewhat from the natural frequency as any real system is damped. Hereinafter however—as is also frequently the case in the literature—the terms resonance frequency and natural frequency are used synonymously.
The most important natural frequency of the ultrasonic oscillating unit is generally the natural frequency at which a standing longitudinal oscillation with wave nodes and wave antinodes is produced in the ultrasonic oscillating unit. In that case a respective antinode occurs at the ends of the sonotrode.
The converter which generates the corresponding ultrasonic excitation frequency is connected to one of the ends. Optionally connected between the converter and the sonotrode is a booster or amplitude transformer which changes the amplitude of the ultrasonic oscillation but not the frequency. The provision of a booster does not influence the natural frequency of the sonotrode and thus the position of the oscillation nodes of the longitudinal oscillation.
For many situations of use the amplitude transformation member and the sonotrode are in one piece, that is to say they can no longer be optically distinguished. In order therefore to distinguish the sonotrode from the amplitude transformation member it is necessary to determine the position of the oscillation antinodes of the pure longitudinal oscillation. The sonotrode always includes the sealing surface. Any portion which extends in the longitudinal direction from oscillation maximum to oscillation maximum and which does not influence the natural frequency of the pure longitudinal oscillation is not part of the sonotrode. In contrast, if such a portion influences the natural frequency of the pure longitudinal oscillation, that is to say it cannot be removed without substantially changing the natural frequency, then it belongs to the sonotrode.
When processing materials by means of ultrasound, in general the material to be processed is positioned between the sonotrode and a counterpart tool (which does not belong to the oscillating structure), which is also referred to as the anvil. The sonotrode in contact with the material to be processed then transmits the ultrasonic energy to the material to be processed which is for example thereby welded or severed. The heat required for plasticising the web of material is generated by the conversion of ultrasound oscillations into frictional energy. By virtue of interface and molecular friction therefore heat is produced, which causes the plastic material to begin to melt.
With most sonotrodes the longitudinal ultrasonic oscillation is used for energy transfer by way of the sealing surface.
There are however also sonotrodes having a sealing surface which is substantially in the form of the peripheral surface of a cylinder, which use the radial ultrasonic oscillation produced transversely relative to the longitudinal direction of propagation of the ultrasonic oscillation, for energy transfer. Those sonotrodes frequently comprise a substantially bar-shaped portion to which the converter and optionally the booster are connected, and a wheel-shaped or bell-shaped portion projecting radially beyond the bar-shaped portion. The wheel-shaped or bell-shaped portion has the sealing surface.
Those sonotrodes generally have two principal natural oscillation modes.
The one natural oscillation mode substantially corresponds to the longitudinal resonance oscillation of the bar-shaped portion. That resonance oscillation is of a relatively great longitudinal oscillation amplitude. However also linked thereto is forced influencing of the material in the transverse direction, that is to say perpendicularly to the bar axis. That forced influencing is expressed in a thickness oscillation which is propagated radially relative to the bar axis. The oscillation amplitude of the thickness oscillation is relatively low, the result thereof being that the major part (more than 90%) of the oscillation energy in the oscillation system is contained in the longitudinal oscillation.
The other natural oscillation mode substantially corresponds to the resonance of the radial oscillation of the wheel portion. Linked thereto is a comparatively slight (forced) oscillation in the longitudinal direction. The major part (generally more than 90%) of the oscillation energy in the oscillation system is contained in the radial oscillation in that natural oscillation mode.
In the case of rotational welding the second natural oscillation mode is used as a relatively great radial oscillation can be produced in the wheel portion of the sonotrode by producing a relatively small longitudinal oscillation in the bar-shaped portion of the sonotrode.
Thus, sonotrodes having a sealing surface in the form of a cylinder surface are known, which are used for continuous ultrasonic treatment of moved webs of material. In operation those sonotrodes are rotated about their longitudinal axis so that the sealing surface in the form of the cylinder surface moves at substantially the same speed as the web of material to be processed. Thus in the case of those sonotrodes there is only ever a small part of the sealing surface, in contact with the web of material.
The ultrasonic unit, that is to say the oscillating structure, must be kept suitably positioned relative to the material or web of material to be processed. In that respect, high demands are to be made on the holder of the ultrasonic unit as on the one hand the holder must ensure that the sealing surface is held exactly at its position relative to the material to be processed while on the other hand the ultrasonic unit must remain oscillatable so that only a negligible part of the ultrasonic energy is transmitted into the machine stand.
As a standing ultrasonic wave is formed within the oscillating structure in the longitudinal direction, that is to say oscillation nodes and oscillation antinodes are formed, oscillating structures are frequently supported at the oscillation nodes of the longitudinal oscillation. Decoupling of the thickness oscillation, that is to say the transverse oscillation, is generally effected by using O-rings. In addition, the holding effect is generally provided in the region of the oscillating structure where only oscillations of low amplitude are to be expected.
The holder using O-rings admittedly provides very good oscillation decoupling but it is very soft in the radial and axial directions so that precise positioning of the sonotrode relative to the material to be processed cannot be implemented as in the excited condition or when coming into contact with the material to be processed the sonotrode can ‘go away’.
Rigid mountings have occasionally also been proposed. Thus for example FIG. 6 of EP 1 455 957 B1 shows a rigid Z-shaped mounting arrangement for a rotational sonotrode. By means of that mounting arrangement, relatively good oscillation decoupling can be achieved with at the same time good stiffness for the holder in the axial direction. It will be noted however that the holder is relatively soft in the radial direction so that any bending forces occurring can only be limitedly accommodated. In addition the Z-shaped mounting arrangement takes up a relatively large amount of space in the radial direction, which in turn limits the possible uses of the rotational sonotrode.
FIG. 13 of EP 1 455 957 already discloses a rotational sonotrode with two Z-shaped mounting arrangements disposed at both sides of the sonotrode. In this case also relatively good oscillation decoupling with good stiffness in the axial direction can be achieved. The mounting arrangement is very soft in the radial direction and although bending forces can be better accommodated by virtue of the two-sided mounting arrangements, in comparison with the single-sided mounting configuration, the ultrasonic unit can nonetheless suffer from temporary flexing, with corresponding adverse consequences for the welding result. In this construction also the amount of space required at the periphery of the ultrasonic unit is relatively great by virtue of the Z-shaped holder.
In addition WO 99/059760 shows a mounting arrangement which engages the oscillation maxima. For that purpose a thin metal disk is clamped between two oscillating elements. The disk in turn is of a special configuration to carry the high oscillation amplitude and not transmit it to the radially outwardly disposed mounting location. That disk mounting arrangement exhibits relatively good stiffness in the radial direction but nonetheless the disk can only be fastened to mounting locations of low oscillation amplitudes and offers only a low level of axial stiffness for the oscillating unit. In addition the space required in the radial direction is very great. Furthermore the thin mounting disks can only limitedly carry radial forces and are susceptible to stress cracks.
For that reason in DE 10 2005 063 230 B3 the applicant has already proposed an ultrasonic oscillating unit having two boosters arranged in succession in the axial direction and on to which a sleeve-shaped holder is fitted, which is respectively supported at flanges which project radially beyond the boosters and which are arranged at oscillation nodes of the oscillating unit. That structure provides for a very flexurally stiff mounting arrangement. In addition very high radial forces can be carried by that mounting arrangement, while oscillation decoupling at the same time is very good. The sleeve holder also has the advantage that the structural height in the radial direction is very slight. There is however the disadvantage that the structural height increases in the axial direction due to the provision of two boosters.